Building Blocks of DNA Found in Meteorites from Space

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Building Blocks of DNA Found in Meteorites from Space

By Charles Q. Choi
Published August 09, 2011
| Space.com

NASA's Goddard Space Flight Center/Chris Smith
Meteorites contain a large variety of nucleobases, an essential building block of DNA.

The components of DNA have now been confirmed to exist in extraterrestrial meteorites, researchers announced.
A different team of scientists also discovered a number of molecules linked with a vital ancient biological process, adding weight to the idea that the earliest forms of life on Earth may have been made up in part from materials delivered to Earth the planet by from space.
Past research had revealed a range of building blocks of life in meteorites, such as the amino acids that make up proteins. Space rocks just like these may have been a vital source of the organic compounds that gave rise to life on Earth.
Investigators have also found nucleobases, key ingredients of DNA, in meteorites before. However, it has been very difficult to prove that these molecules are not contamination from sources on Earth.
"People have been finding nucleobases in meteorites for about 50 years now, and have been trying to figure out if they are of biological origin or not," study co-author Jim Cleaves, a chemist at the Carnegie Institution of Washington, told SPACE.com.
To help confirm if any nucleobases seen in meteorites were of extraterrestrial origin, scientists used the latest scientific analysis techniques on samples from a dozen meteorites — 11 organic-rich meteorites called carbonaceous chondrites and one ureilite, a very rare type of meteorite with a different chemical composition. This was the first time all but two of these meteorites had been analyzed for nucleobases.
The analytical techniques probed the mass and other features of the molecules to identify the presence of extraterrestrial nucleobases and see that they apparently did not come from the surrounding area.
Two of the carbonaceous chondrites contained a diverse array of nucleobases and structurally similar compounds known as nucleobase analogs. Intriguingly, three of these nucleobase analogs are very rare in Earth biology, and were not found in soil and ice samples from the areas near where the meteorites were collected at the parts-per-billion limits of their detection techniques.
"Finding nucleobase compounds not typically found in Earth's biochemistry strongly supports an extraterrestrial origin," Cleaves said.
"At the start of this project, it looked like the nucleobases in these meteorites were terrestrial contamination — these results were a very big surprise for me," study lead author Michael Callahan, an analytical chemist and astrobiologist at NASA Goddard Space Flight Center, told SPACE.com.
Lab experiments showed that chemical reactions of ammonia and cyanide, compounds that are common in space, could generate nucleobases and nucleobase analogs very similar to those found in the carbonaceous chondrites. However, the relative abundances of these molecules between the experiments and the meteorites differed, which might be due to further chemical and thermal influences from space.
This findings reveal that meteorites may have been molecular tool kits, providing the essential building blocks for life on Earth, Cleaves said.
"All this has implications for the origins of life on Earth and potentially elsewhere," Callahan said. "Are these building blocks of life transferred to other places where they might be useful? Can alternative building blocks be used to build other things?"
In a different study, researchers discovered molecules that make up key parts of a vital biological pathway, the citric acid cycle, in a number of carbonaceous chondrites.
The citric acid cycle is "thought by many experts to be among the most ancient of biological processes," study co-author George Cooper, a chemist at NASA Ames Research Center, told SPACE.com. "One function of this cycle is respiration, when organisms give off carbon dioxide."
"It is always exciting to find extraterrestrial and ancient 4.6 billion-year-old organic compounds that might have had a role in early life," Cooper added.
Cleaves, Cooper and their colleagues detailed their findings in two studies online Aug. 8 in the Proceedings of the National Academy of Sciences.
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Billions of years ago, comets may have ferried life-sustaining water to our planet's surface, but that may not be all that we should thank these dirty snowballs for. Researchers are simulating comet impacts to see if they might help proliferate the left-handedness in molecules that life on Earth depends upon.
There is evidence from meteorite studies that amino acids may have been delivered to Earth from space.
"There is interest in how these building blocks came to be on primordial Earth," says Jennifer Blank of the SETI Institute.


She and her colleagues study comets as a second avenue for depositing these biological compounds on Earth. Their current work, which is supported by NASA's Exobiology and Evolutionary Biology Program, is looking at how the fire and brimstone of a comet impact may benefit the formation of complex molecules of a particular handedness.
Primordial Lab
Life on Earth uses 20 amino acids to build up the thousands upon thousands of different proteins that perform a myriad of cell functions. Astrobiologists often focus on the origins of amino acids in order to understand where life may have come from.
One of the first experiments aimed at reproducing the primordial Earth and its chemistry was undertaken by Stanley Miller in 1953. He was able to synthesize amino acids using lightning-like discharges in a reducing atmosphere of methane, ammonia and water ? similar to what exists on Jupiter.
Since that pioneering work, researchers have come to believe that Earth's early atmosphere was in fact more oxidative, containing mostly nitrogen and carbon dioxide.
"Without the reducing atmosphere, the Miller mechanism becomes much less efficient at producing amino acids," Blank says.
One way to get around this is to make the amino acids in space and have them come crashing down on-board meteorites and comets. There is ample evidence that meteorites carry amino acids. And just recently, an amino acid was discovered in comet material brought back by NASA's Stardust spacecraft.
Blank and her colleagues were curious as to what happens to these biomolecules when the "space capsule" they are riding in smacks into the Earth.
The team has focused their work on comets, rather than meteors. Although comets are less prevalent in the inner solar system, they have a few possible advantages over their dry rocky counterparts when it comes to delivering biologically relevant material to a planet's surface.?
First of all, a comet impact is thought to be less harsh than that of a meteorite because comets are less dense, which means their impact generates lower temperatures and pressures. Blank says that the blow would be further softened on a comet arriving at an oblique angle.
The second advantage of comets is that they carry water, which is key for the chemical reactions that beget life. When the comet lands, its ice melts, forming a little puddle near the crash site.
"Comets give you all the ingredients, like a compact evolution kit," Blank says.
Of course, the primordial Earth was stocked with its own water, but "if a comet or meteor were to land in the ocean, any interesting chemistry would quickly be diluted away," says co-investigator George Cooper of NASA Ames. A comet impact on dry land would give the organic molecules on board the chance to amplify their numbers in the localized puddle.
Like Shooting Comets in a Barrel
To simulate a comet hitting pay dirt, Blank and her colleagues fire a bullet into a metal container the size of a can of beans. In this scenario, the container is the comet and the bullet is the hard ground. Inside the container is a small chamber about as big as a quarter, in which the scientists place a liquid sample of organic molecules.
"It's not super high-tech, but it is rather involved as far as the structural complexity is concerned," Blank explains.?
She and her colleagues take special care to ensure that the metal container doesn't leak from the impact. Afterwards, they carefully drill down to the chamber and draw out their "shocked" liquid sample.
In 2001, the team reported that amino acids placed in the comet simulator were still intact following the impact, which surprised other scientists.
"It's the coolest thing," Blank recounts. "People told us, 'Nothing is going to survive, so why should we fund you?'"
Normally, the 1,000-degree-temperatures inside the smashed "comet" would destroy any amino acids. But Blank believes the temperature rises and falls too fast for the molecules to react. There is also enormous pressure of 10,000 atmospheres that may be preventing the breakdown of compounds.
However, the amino acids did more than just survive the crash. They also started bonding together to form short chains up to 5-amino-acids long.
This comet-induced bonding may have played a role in the origin of life. Typically, there is an energy barrier that prevents amino acids from latching together. Indeed, organisms require enzymes to overcome this barrier when putting together their proteins. But enzymes themselves are proteins, so there is a bit of a chicken-and-egg problem: how do you build up proteins before you have proteins to help build them up?
It is perhaps conceivable that a comet impact fused together the first rudimentary protein pieces (called "peptides") and thereby got the whole ball rolling.
Blank's group is now running simulations to see if they can model how the energy barrier to amino acid bonding changes under the high temperature and pressure of a comet impact.
Molecular Crash-Test Dummies
The scientists are also planning to do more comet crash tests. They will be looking at sugars, which play an important part in the structure of DNA and RNA. And they will be looking at amino acids again, this time studying whether the handedness of comet passengers might be affected by the impact.
In regard to the handedness, Blank thinks there might be a difference in how the amino acids hook up together during the impact. Left-handed amino acids may form chains more readily with other left-handed amino acids, rather than with right-handed ones.
Such a preference, if it exists, might be able to enhance a slight overabundance of one hand (a so-called enantiomeric excess) in the original comet material. This might explain why organisms only use left-handed amino acids to form proteins.
"It will be a great discovery if they can get definite evidence as to formation of sugars, peptides, or enantiomeric excess," says Yoshihiro Furukawa of Tohoku University in Japan, who was not involved with this work.
He says the one concern will be contamination of the sample with the left-handed biology we are already familiar with. He suggests using amino acids made with carbon-13, so that any subsequent contamination with normal carbon-12-based amino acids will be easy to detect.